Comparison of the Electronic Properties of Diarylamido-Based PNZ Pincer Ligands: Redox Activity at the Ligand and Donor Ability Toward the Metal

This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR₂) substituents in the <i>ortho</i>-positions (PNP ligands) or by a combination of a phosphino and an iminyl (−CHNX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C_2v-symmetric PNP ligands with identical side −PR₂ donors, (b) C_s-symmetric PNP′ ligands with different −PR₂ side donors, and (c) PNN ligands containing a −PⁱPr₂ side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)­PdCl, (PNZ)­Pd­(OAc), and (PNZ)­RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)­NiCl and (PNP)­PtCl were also studied. The (PNZ)­MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the <i>E</i>_1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)­PdCl complex containing methoxy substituents <i>para</i>- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)­RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of <i>E</i>_1/2 values for (PNZ)­PdCl and ν_CO values for (PNZ)­RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration